The present invention relates to a power unit for a motorcycle such as a scooter-type vehicle.
With respect to a motorcycle such as a scooter-type vehicle, there has been proposed a power unit, which is composed of an engine unit for generating power in combination with a transmission unit, which is separated from the engine unit and includes a belt drive-type transmission device such as a CVT (Continuously Variable Transmission) therein. According to such a power unit, it is possible not only to manufacture the respective units mentioned above in different manufacturing places, thus improving productivity and maintenance property of the power unit, but also to make a change in combination of the units, coping with many types of motorcycle.
In such a power unit, a crankshaft and a countershaft are rotatably supported in a casing of the engine unit and a transmission input shaft and a transmission output shaft are rotatably supported in parallel to each other in a casing of the transmission unit to define a belt drive-type transmission device so that the transmission input shaft of the transmission unit is connected to the countershaft of the engine unit so as to be rotatable together with the countershaft.
Heat is generated through internal friction of a belt due to its expansion and contraction as well as contact friction between the belt and pulleys during operation of the belt drive-type transmission device of the transmission unit.
Accordingly, the pulley placed on the input side or the output side in the belt drive-type transmission device is provided with a cooling fan so that the cooling fan is rotatable together with the pulley, with the result that outside air serving as cooling air is sucked into the casing through a suction duct provided thereon, thus introducing the cooling air into the casing to carry out a cooling process.
There however exist problems that intense heat generated by the engine unit is transmitted through the casing and the countershaft to the transmission unit, even when the cooling fan cools frictional heat generated by the belt drive-type transmission device per se, making it impossible to provide a sufficient cooling effect of the transmission unit, thus deteriorating durability of the belt as well as the belt drive-type transmission device.
The object of the present invention, which was made to solve the above-described problems, is to provide a power unit for a motorcycle, which comprises an engine unit and a transmission unit and permits to isolate heat generated by the engine unit from the transmission unit and enhance a cooling operation in the transmission unit, thus improving durability of a belt drive-type transmission device.
This object can be achieved according to the present invention by providing, in one aspect, a power unit for a motorcycle comprising:
an engine unit having a casing in which a crankshaft and a countershaft are rotatably supported in parallel to each other;
a transmission unit having a casing in which a transmission input shaft and a transmission output shaft are rotatably supported in parallel to each other to define a dry belt drive-type transmission device, said transmission input shaft of the transmission unit being connected to said countershaft of the engine unit so as to be rotatable together with said countershaft;
a cooling fan chamber provided at a joined portion of the casing of said engine unit to the casing of said transmission unit, said cooling fan chamber communicating with inside and outside of the casing of the transmission unit; and
a cooling fan disposed in said cooling fan chamber so as to rotate together with a connecting portion of said countershaft with said transmission input shaft in a vicinity of said connection section;
Wherein said cooling fan introduces cooling air into the casing of the transmission unit.
Such a structure of the power unit for a motorcycle causes cooling air to flow always during operation of the power unit in the cooling fan chamber, which is provided between the casing of the engine unit and the casing of the transmission unit. Accordingly, it is possible to isolate heat generated by the engine unit from the transmission unit and enhance the cooling operation in the transmission unit, thus improving durability of the belt drive-type transmission device.
In preferred embodiment of the above aspect, an additional cooling fan is also provided on the transmission output shaft so as to rotate together with the transmission output shaft, an opening communicating with outside of said transmission unit is provided in a vicinity of said additional cooling fan, one of said additional cooling fan and said cooling fan provided rotatably together with said countershaft and said transmission input shaft serves as a inlet fan and an other thereof serves as a suction fan.
Such a structure makes it possible to increase flow velocity and flow rate of the cooling air in the casing of the transmission unit, thus remarkably improving the cooling operation in the transmission unit, in comparison with a case where a single fan is provided.
More preferred embodiment of the above aspect, said cooling fan provided rotatably together with said countershaft and said transmission input shaft can be made of material having a high thermal conductivity.
This structure makes it possible to radiate the most of heat, which is generated by the engine unit and is apt to be transmitted to the transmission unit through the countershaft, by means of the cooling fan, thus isolating heat from the transmission unit.
Further preferred embodiment of the above aspect, said joined portion of the casing of said engine unit to the casing of said transmission unit is provided with a gasket held therebetween, said gasket being made of thermal insulating material. The above-mentioned gasket isolates heat of the casing of the engine unit from the casing of the transmission unit.
More preferred embodiment of the above aspect, said countershaft and said transmission input shaft are connected to each other through a flexible joint. Such a flexible joint absorbs torque fluctuation caused from the countershaft so as to prevent sudden slip from occurring between the belt and the pulleys of the belt drive-type transmission device. An excessively large frictional force therefore does not occur, improving the cooling operation in the transmission unit.
Also, said cooling fan provided rotatably together with said countershaft and said transmission input shaft is stationarily secured to a side of said countershaft; and said flexible joint is provided between said cooling fan and said transmission input shaft.
Such a structure makes it possible to radiate the most of heat, which is generated by the engine unit and is apt to be transmitted to the transmission unit through the countershaft, by means of the cooling fan, and isolate residual heat from the transmission input shaft by means of the flexible joint, thus effectively preventing heat generated by the engine unit from being transmitted to the transmission unit.
Further preferred embodiment of the above aspect, at least one portion of said flexible joint is made of thermal insulating material. Such a structure makes it possible to surely isolate heat, which is generated by the engine unit and is apt to be transmitted to the transmission unit through the countershaft and the transmission input shaft, by means of the flexible joint, thus further enhancing the cooling operation in the transmission unit.